Regeneration of caustic soda-sodium sulfide cooking liquor



Patented Sept. 6, 1949 REGENERATION OF CAUSTIC SODA-SODIUM SULFIDE COOKING LIQUOR Robert R. Fuller, Tuscaloosa, Ala., and Vernon Woodside, Niagara Falls, N. Y., assignors to Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Application August 13, 1946, Serial No. 690,342

. 3 Claims. 1

This invention relates to the preparation of aqueous solutions of caustic soda and sodium sulfides such, for instance, as used in the cooking or digesting of cellulosic fibers, or the like, in the preparation of pulp for the manufacture of paper. The invention provides an improved method whereby such solutions or cooking liquors may be more readily and economically prepared. It will be understood that the invention is of general application to the preparation of such solu- 1 tions and is not dependent upon the intended use of the solution.

Seed flax straw, for instance, has been used extensively in the manufacture of paper for special purposes requiring a high degree of purification water of prescribed amounts of caustic soda and sodium sulfide.

Though this customary, practice has been found generally effective, it is subject to the economic disadvantage of high cost of chemicals employed. In such operations large amounts of caustic soda and sodium sulfide are used. The sodium sulfide, particularly, is a relatively expensive chemical and materially adds to the cost of the operation. The causticsoda, though usually less expensive than sodium sulfide, is still an item of considerable expense.

It has heretofore been proposed to prepare digestion liquors for use in the preparation of paper pulp by separately adding caustic soda and elemental sulfur directly to the digester containing the cellulosic material and water. The substitution of sodium carbonate and lime for the caustic soda in such operations has likewise been suggested.

In our co-pending application Serial No. 491,056 (now abandoned), filed June 16, 1943, of which the present application is in part a continuation, we have specifically described a highly advantageous application of the present invention to the pulping of wood by a process somewhat resembling the kraft process.

In paper manufacture, by the kraft process, for instance, wood chips are cooked in an aqueous solution containing as essential ingredients, sodium hydroxide and sodium sulfide. Such solutions also usually contain minor proportions of sodium polysulfide. Following the cooking or digesting period the spent liquor is conventionally regenerated by eliminating water to form the so-called *black liquor, carbonizing the organic III carbonate.

matter therein, and smelting the salt mixture thus formed, dissolving the smelt in water, and causticizing the resultant liquor. The liquor comprising the dissolved smelt is commonly called green liquor, and the causticized green liquor is commonly called white liquor.

Make-up soda and sulfur to replace that consumed or lost in the cooking operation are conventionally added to the black liquor or to the carbonizing and smelting operation as sodium sulfate, which is reduced to sodium sulfide by reaction with the organic material.

It has also been proposed to add make-up soda and sulfur separately either as sodium carbonate and elemental sulfur to the green liquor prior to the causticizing operation, or as sodium carbonate to the green liquor and as elemental sulfur to the white liquor. Sodium carbonate added to the green liquor is converted to sodium hydroxide in the causticizing operation.

At an elevated temperature, elemental sulfur will react with caustic soda to form sodium-sulfur compounds consisting of sodium monosulfide and considerable proportions of polysulfides. Howi ever, we have found that this reaction proceeds relatively slowly and that, under ordinary conditions, the passing of the added elemental sulfur into solution is so time-consuming as to detract materially from the economic advantage of its use over the use of sodium sulfide as such.

In accordance with our present invention, the cooking liquor is prepared by dissolving in water alone or in green liquor an intimate admixture of elemental sulfur and soda ash and causticizing the solution, as by the further addition of lime in suflicient quantities to convert the soda ash to caustic soda.

, We have discovered that the dissolving of the elemental sulfur is very substantially accelerated if it be added in intimate admixture with sodium The admixture may with advantage be prepared by intimately admixing mechanically the soda ash and sulfur in appropriate proportions. However, we have obtained particularly beneficial results by adding the soda ash and sulfur as an intimate admixture'agglomerated by fusion as described in United States Letters Patent No. 2,223,631 to Robert B. MacMullin. There is described in said patent a fused admixture consisting essentially of sodium carbonate and sulfur in proportions by weight of about 10 parts soda ash and 3 parts sulfur intimately admixed and agglomerated by fusion of the sulfur in a nonoxidizing atmosphere and in which the two constituents are chemically uncombined, these proportions being substantially those in which the constituents combine to produce sodium sulfide. This agglomerated admixture has been referred to in said patent as synthetic saltcake and, for convenience, 15s so.designatedumerein.

Though the,synthetics-saltcakeidescribediin said patent, in which the soda ash and sulfur are present in molecularly combining weights to produce NazS, has been found particularly desirable for use in accordance with-ourpresent;inVent-ion,' it will be understood that the proportionspf'soda ash and sulfur may be variedzconsiderablyt to meet the requirements of partioiilarsoperations.

In the preparation of 'the syn thetic saltcake, the soda ash and sulfur, preferably of relatively small, uniform particle size, ares advantageously admixed at room temperature, prior to agglomeration by fusion, and then heated in a non-oxidizing 7 atmosphere, with continued agitation,'--to'a -temperature above the melting point of the sulfur i (120 210. :but v bielowiztlaatsatewhich:substantial: re- ..xaeti'on-betweeiiethe cnnstituentsnwilhmsult. rPrefaaerablwthe agitation zisacnntinued? during: cooling soaasltoapreventathesfnrmatiombf ilargeslumps.

ll-he proportions-bimodal.ashiand' sulfur usedwill depend upon thenprpportion 'ofzcausticcsodarand -;sodium:sulfide-requiredxin the. cookingiliquore and .whetherztherentirercanstic sodaszonstituent-ioftthe i :cookingl liquon isiitoilbe supplied xbyathe *sodarash- :sultur admixture. :1 It :willnbe observedsthatwthe :proportions previously notedyare suclr-as to result in the formatiombfi Nazsrwithout provision forad- :Editional causticrsoda: requirement 01": the: cooking :lliquor. Additional soda; ash: maycbe separately xadded or addedqin admixtureswith: the sulfur to supply-the :entire: caustic soda requirement. A further alternative is toiadd ianyr further require- :mentaof. caustic soda. 3182811011.

a Welhave' foundsalsoithatsthez dissolvingmofsthe elemental sulfur is stilt-'iurthenexpeditedyby the presencesofi asmalkamount-of ipren-formed sodium --sulfide. acomrenientsourcezofithe sodium; sul- .-:fide used for -thismurposeraiportion of the cook- .-ing liquor fromaa rpreviousr pperationnnay. :be-used -withv-advantage. :i-Ztn :startingnupwthe" operation, wheneno previously; rusedecookingcliquoreis-:available, SOdilllllSlllifidGzfi'OHIi any outside source may beausedand, if :desired,-sod-imn.sulfide from an .extraneous; source mayibes-usedthroughoutto .ac-

celeratethe dissolving/Mattie sulfur. i-rHowever; a ismalliamount'oftthe preyiouslyaused cooking liq- -.uorhas been -found .atoabet;entirely-satisfactory forrthis-.-purposeaandeiseavailable.withouteextra \-cost. 1 l

:I-he amountofi sodiumsulfide added tolaccelermate the:dissolving-=ot-.-the. smut-sis: not particularly critical. Amounts substantially less'.'than$1%, v. .-g.enerally;- assmall rtraction. of; 1% :say.-.0.1 %=.to as eloweasa 005%;zoreeivensless has beenfoundstovbe -.--efiective;

aeannot. explain-i-witheentainty the 1 chemical or' physicalreactions-resulting in-ithetd-issolving of -the-.elementaL.sulfumornjust how and why these wreaotionsa are-.a'fiectedsbyrthe conditions contemplated bye ourvinvenltion. The. dnventioneisrsnot -.-predicated.upon anyttheormaslto-ithe reasons i for srthe :improvedz results; :obtained. andmthe utility. of .the*1nvention-isiunaffectedrby-ithesabsence. of such explanation.

e=have-observed however, that-when sulfur in -the form of flowerse-ofssuliur iszseparatelysadded vtO thealiquorpadarge; portiona-thereofs. at least, floats on thezsur/faeewof the liquor with .considerablepersistence but ,-that,-iwhen -thesulfur.is;added in; mtimate adm-ixture withzthefsodafash particularly when a fused admixture of the soda ash and sulfur, such as previously described, is used, the sulfur passes readily into solution. It ap: pears likely that, by reason of the close proximity 5 .of...the.sulfur.-to..the soda during the dissolvmg: of the. latter, the; dissblution bf the former with the formation of more or less monosulfide (and sodium polysulfide is accelerated. We cannot account for the further acceleration of this action 5 1 -by the presence ofiipreformed NazS.

In--the-preparation of the sulfide cooking liq-- uor,.inaccordancewith our present invention, an .intimate admixture of the soda ash and sulfur in -fappropriateeproportions may be admixed with wa- 15 ter zinsamountsudepending upon the particular concentration of. the liquor required. As pre- .vio.usly noted, 'it" is not necessary that the full .;tionmay bemade either before or after the. causti- ..,cization. T'IThecausticizing .of'the solution results ,.in,ftheformation ofcalcium carbonate whichmay Jnbe. removed'from'the 'soliition by' precipitation" or, mfin many instances, may be'allowed to remain'in the liquor.' A 7 .Cookingliquors conventionally used in di estion .mczeraitions. .sucl'r as referred to .herein" ordinarily 7 contain caustic. soda and sodium sulfide; J :in aamounts. equivalentjto about"10t.to 12 pounds-pf .caustic soda andt3'to'6 poundsmf sodium sulfide per1100 pounds of dry fiber'treated.

.The. proportions and, concentrations. of caustic soda-.arid ,sodium. sulfides ,in' the cooking liquor 50. may. vary,considerably;;depending upon the particular cellulosic material being digested and ...anduother operating conditions employed,.including' temperaturaand'duration ofthe cooking and .also thede'sired. characteristics 'oftheproduct. J.In .carrying..out.,-the process embodying theyim- ,provements. of;ou1:, present, invention, onejshould sfirstdetermine'theproportions.and, concentrations ofgcaus'tic sodar. andsndium sulfides to-be employed under the selected conditions of operation. 'I'I'he v proportions and concentrationsmay be'determined in .accordance ewith.conventional. .cookinspram ,tice .as; is. wel1 .zunde'rstoodinithe. industry. Hav- .ing-decidedwhat.proportionsJand.concentrations ..-of..caustic-.soda...and sodium sulfideaare to..be.em- 05 ..ployed, the. amounts and! relative, proportions of sodium carbonate andrelemental sulfur.to=be.used .-..may.-.be'- determined .in the; customarymanner, as

thy-analysis of l. :the resultant liquor .by conven- .tionalmethods. 'aWehave found, however; that, in thefdigestion eoperatiomsa, substantial; proportion. of the polysulfide formed ,-by the reaction of. the, elemental :--sulfur.-with..the causticaappears-lto react with vthe .iNaoHvto-form .the ,monos'ulfide. and, .by. this. re- .75saction aiportionpt the-caustic sodais consumed.

equivalent of the NaOH requirement of the liquor Therefore, the analysis of the digestion liquor prior to use is not a wholly accurate guide with respect to causticity of the cooking liquor in the digester. A more accurate guide for determining the amounts and proportions of sodium carbonate and sulfur to be added is the analysis of a sample of the liquor from the digester after a brief period of the cooking.

The concentration of the cooking liquor and proportions of constituents thereof prepared in accordance with our present invention may with advantage be those customarily used. For instance, the amount of water used in the preparation thereof may be about two to ten times the dry weight of the fiber to be treated. For each 100 pounds of fiber, 1 to 4 pounds of sulfur, to pounds of soda ash. and 4 to 10 pounds of lime may be used, depending upon the desired causticity and sulfidity of the liquor.

In our above-noted co-pending application we have described an operation in which the soda ash-sulfur mixture is added to the green liquor resulting from the dissolving of the smelt from a previous cooking operation. In that operation conditions are adjusted and controlled so as to maintain a particularly high ratio of sodium polysulfide to sodium monosulfide in the cooking liquor. The present invention is especially applicable to operations in which such conditions are maintained but its use is not so restricted.

In commercial pulping operations in which elemental sulfur is used, the sulfur is usually used in lump form rather than the more expensive and less readily handled flowers of sulfur. The

solution of elemental sulfur in either form in either water alone or in green liquor is a relatively troublesome time-consuming operation. The advantages derived. from our present invention will appear from the following specific illus trations of the invention as applied to the addition of the soda ash-sulfur mixture to a green liquor. For comparison, the time required to dissolve the sulfur when separately added in lump form and as flowers of sulfur to the green liquor is included.

In order more accurately to determine and standardize conditions, the green liquor used in each of these tests was prepared by dissolving sodium sulfide and sodium carbonate in water to form a solution containing grams per liter of sodium sulfide and 140 grams per liter of sodium carbonate. In each test the resultant green liquor was heated on a heat plate to 80 C. before the additional ingredients were added, and was stirred by means of a constant speed stirrer at relatively low speed in order to approximate as nearly as possible the agitation obtained in a dissolving tank in commercial operation. In each instance the proportions of soda ash and elemental sulfur were 10:3. Also, in each instance the point indicated as that when substantially complete solution was attained was that when substantially all of the solid had passed into ill) solution and the liquor between any remaining solid particles was clear.

Where synthetic salt cake such as previously described herein was used, substantially complete solution of the solid was attained in 2 minutes. Repeated tests confirmed this result. Where soda ash and finely divided elemental sulfur in unfused intimate admixture were added, substantially complete solution was attained in '7 minutes. Where soda ash and finely divided sulfur were separately added simultaneously, only about three-fourths of the sulfur had passed into solution at the end of 11 minutes, and about 20 minutes was required for substantially complete solution of the sulfur. Similarly, where soda ash and lump sulfur were simultaneoulsy added, about 20 to 23 minutes was required to attain substantially complete solution of the solids.

We claim:

1. In. the preparation of aqueous solutions of caustic soda and sodium sulfide for use in the manufacture of paper pulp by the kraft process, in which the make-up sulfur and sodium are added to the green liquor as elemental sulfur and sodium carbonate, respectively, and the green liquor then causticized, the improvement which comprises adding the sulfur and sodium carbonate in intimate admixture with one another in amounts to regenerate the cooking liquor.

2. In the preparation of aqueous solutions of caustic soda and sodium sulfide for use in the manufacture of paper pulp by the kraft process, in which the make-up sulfur and sodium are added to the green liquor as elemental sulfur and sodium carbonate, respectively, and the green liquor then causticized, the improvement which comprises adding the sulfur and sodium carbonate in intimate fused admixture with one another in amounts to regenerate the cooking liquor.

3. In the preparation of aqueous solutions of caustic soda and sodium sulfide for use in the manufacture of paper pulp by the kraft process, in which the make-up sulfur and sodium are added to the green liquor as elemental sulfur and sodium carbonate, respectively, and the green liquor then causticized, the improvement which comprises adding the sulfur and sodium carbonate in intimate fused admixture with one another in amounts to regenerate the cooking liquor, the admixture comprising by weight about ten parts of soda ash to three parts of sulfur.

ROBERT R. FULLER. VERNON WOODSIDE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,007,024 Richter s July 2, 1935 2,223,631 MacMullin Dec. 3, 1940 

